High cost and unavailability of platinum catalyst is the main hurdle to commercialize polymer electrolyte membrane fuel cells (PEMFC). Since platinum based catalysts are inevitable for PEMFC cathode in current situation, a material showing comparable or higher activity to platinum based catalyst with low cost is a need and a challenging task. Carbon allotropes are proven to be best candidates to improve kinetically slow oxygen reduction reaction (ORR). Most of the carbon morphologies are lacking high surface area with good electrical conductivity. Both surface area and electrical conductivity are inevitable properties of a material for ORR. But both are complimentary to each other. If surface area of the material is increasing, simultaneously its conductivity will come down. So it is important to provide a material with both high surface area and conductivity.
Graphene is reported to have theoretical surface area more than 2000 m2/g. But, surface area of synthesized graphene is less than 1000 m2/g. Different methods have been employed to improve the surface area of graphene, but it reduces electrical conductivity of graphene. Other carbon morphologies such as carbon nanotubes, nanofiber and such like have low surface area, but have good conductivity. Recently, heteroatom doped, mainly nitrogen doped carbon morphologies are proving to be an alternative to Pt based ORR catalyst. But a nanostructure with high surface area (for proper mass diffusion) and high electrical conductivity along with nitrogen doping still remains a hurdle to the scientific community.
Article titled “Flowing nitrogen assisted-arc discharge synthesis of nitrogen-doped single-walled carbon nanohorns” by L Sun et al. published in Applied Surface Science, 15 Jul. 2013, Volume 277, Pages 88-93 reports nitrogen-doped single-walled carbon nanohorns (N-SWCNHs) synthesized by a flowing nitrogen assisted arc discharge method at atmospheric pressure in a tubular reactor. X-ray diffraction and thermo gravimetric analysis have revealed their high quality. Scanning electron microscopy and transmission electron microscopy examinations have shown that N-SWCNHs have typical spherical structure with a diameter of 40-80 nm. Oxidation treatment suggests the opening of cone-shaped caps of N-SWCNHs. The FT-IR and X-ray photoelectron spectroscopy analysis indicate that most of the nitrogen atoms are in N-6, N-5, and triple-bonded —CN bonding configuration present at the defect sites or the edges of graphene layers.
Article titled “Single-walled carbon nanohorns and their applications” by S Zhu et al. published in Nanoscale, 2010, 2, 2538-2549 reports Single-walled carbon nanohorns (SWCNHs) horn-shaped single-walled tubules with a conical tip. They are generally synthesized by laser ablation of pure graphite without using metal catalyst with high production rate and high yield, and typically form radial aggregates. SWCNHs are essentially metal-free and very pure, which avoids cumbersome purification and makes them user-friendly and environmentally benign. Currently, SWCNHs have been widely studied for various applications, such as gas storage, adsorption, catalyst support, drug delivery system, magnetic resonance analysis, electrochemistry, biosensing application, photovoltaics and photoelectrochemical cells, photodynamic therapy, fuel cells, and so on. This review outlines the research progress on SWCNHs, including their properties, functionalization, applications, and outlook.
Article titled “Facile Synthesis of Nitrogen-Doped Graphene via Pyrolysis of Graphene Oxide and Urea, and its Electrocatalytic Activity toward the Oxygen-Reduction Reaction” by Z Lin et al. published in Advanced Energy Materials, July, 2012, Volume 2, Issue 7, pages 884-888 reports Nitrogen-doped graphene (NG) as a promising metal-free catalyst for the oxygen-reduction reaction (ORR). A facile and low-cost synthesis of NG via the pyrolysis of graphene oxide and urea is reported. The N content in NG can be up to 7.86%, with a high percentage of graphitic N (≈24%), which gives rise to an excellent catalytic activity toward the ORR.
Article titled “Catalyst-free synthesis of nitrogen-doped graphene via thermal annealing graphite oxide with melamine and its excellent electrocatalysis” by Z H Sheng et al. published in ACS Nano, 2011, 5 (6), pp 4350-4358 reports the electronic and chemical properties of graphene can be modulated by chemical doping foreign atoms and functional moieties. The general approach to the synthesis of nitrogen-doped graphene (NG), such as chemical vapor deposition (CVD) performed in gas phases, requires transitional metal catalysts which could contaminate the resultant products and thus affect their properties. The article discloses a facile, catalyst-free thermal annealing approach for large-scale synthesis of NG using low-cost industrial material melamine as the nitrogen source. This approach can completely avoid the contamination of transition metal catalysts, and thus the intrinsic catalytic performance of pure NGs can be investigated. Detailed X-ray photoelectron spectrum analysis of the resultant products shows that the atomic percentage of nitrogen in doped graphene samples can be adjusted up to 10.1%. Such a high doping level has not been reported previously. High-resolution N1s spectra reveal that the as-made NG mainly contains pyridine-like nitrogen atoms. Electrochemical characterizations clearly demonstrate excellent electrocatalytic activity of NG toward the oxygen reduction reaction (ORR) in alkaline electrolytes, which is independent of nitrogen doping level. The present catalyst-free approach opens up the possibility for the synthesis of NG in gram-scale for electronic devices and cathodic materials for fuel cells and biosensors.
Article titled “Synthesis, characterization and properties of single-walled carbon nanohorns” by K Pramoda et al. published in Journal of Cluster Science, January 2014, Volume 25, Issue 1, pp 173-188 reports single-walled nanohorns (SWNHs) prepared by sub-merged arc discharge of graphite electrodes in liquid nitrogen. The samples were examined by scanning electron microscopy, transmission electron microscopy and Raman spectroscopy. Nitrogen and boron doped SWNHs have been prepared by the sub-merged arc discharge method using melamine and elemental boron as precursors. Intensification of Raman D-band and stiffening of G-band has been observed in the doped samples. The electrical resistance of the SWNHs varies in opposite directions with nitrogen and boron doping. Functionalization of SWNHs through amidation has been carried out for solubilizing them in non-polar solvents. Water-soluble SWNHs have been produced by acid treatment and non-covalent functionalization with a coronene salt. SWNHs have been decorated with nanoparticles of Au, Ag and Pt. Interaction of electron donor (tetrathiafulvalene, TTF) and acceptor molecules (tetracyanoethylene, TCNE) with SWNHs has been investigated by Raman spectroscopy. Progressive softening and stiffening of Raman G-band has been observed respectively with increase in the concentration of TTF and TCNE.
Article titled “Nitrogen-Containing Carbon Nanostructures as Oxygen-Reduction Catalysts” by E J Biddinger et al. published in Topics in Catalysis, October 2009, Volume 52, Issue 11, pp 1566-1574 reports nitrogen-containing carbon nano structure (CNx) catalysts developed by acetonitrile pyrolysis have been studied to better understand their role in the oxygen reduction reaction (ORR) in PEM and direct methanol fuel cell environments. Additional functionalization of the CNx catalysts with nitric acid has the ability to improve both the activity and selectivity towards ORR.
Article titled “Nitrogen-doped carbon nanotubes as catalysts for oxygen reduction reaction” by C Xiong et al. published in Journal of Power Sources, 1 Oct. 2012, Volume 215, Pages 216-220 reports the aligned nitrogen-doped carbon nanotubes (NCNT) with bamboo-like structure are synthesized via thermal chemical vapor deposition using melamine and urea as different nitrogen precursors. Meanwhile, ferrocene is used as catalyst and carbon precursor. The resulting NCNT with melamine (M-NCNT) have shown superior ORR performance in terms of limiting current density and number of electrons transferred. Further characterizations by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy illustrated higher nitrogen content and more defects in M-NCNT compared to that in NCNT with urea (U-NCNT), which indicate the important role of the nitrogen precursor in nitrogen content and structure of NCNT. It is concluded that higher nitrogen content and more defects of NCNT lead to high performance of ORR.
It is observed that simple oxidation creates “nanowindows” on the walls of single walled carbon nanohoms (SWCNH), which enhances its surface area, refer Murata et al in J. Phys. Chem. B 2001, 105, 10210-10216 and Yang et at in J. Am. Chem. Soc. 2006, 129, 20-21.
But, there is no report that discloses doped carbon nanohoms that displays properties that enable their use as anion exchange membrane fuel cell catalyst.